This patch adds backward-compatibility support for the --acls option.
Since the main release has never had ACL support, the trunk doesn't
need this code.  If you want to make rsync 3.0.x communicate with an
older (patched) release, use this.

To use this patch, run these commands for a successful build:

    patch -p1 <patches/acls.diff
    ./configure                         (optional if already run)
    make

based-on: 1ddcdaf3f6808ba53aef9e19f630a18808de22ac
diff --git a/acls.c b/acls.c
--- a/acls.c
+++ b/acls.c
@@ -31,6 +31,7 @@ extern int list_only;
 extern int orig_umask;
 extern int numeric_ids;
 extern int inc_recurse;
+extern int protocol_version;
 extern int preserve_devices;
 extern int preserve_specials;
 
@@ -112,6 +113,18 @@ static const char *str_acl_type(SMB_ACL_TYPE_T type)
 	return "unknown ACL type!";
 }
 
+#define OTHER_TYPE(t) (SMB_ACL_TYPE_ACCESS+SMB_ACL_TYPE_DEFAULT-(t))
+#define BUMP_TYPE(t) ((t = OTHER_TYPE(t)) == SMB_ACL_TYPE_DEFAULT)
+
+static int old_count_racl_entries(const rsync_acl *racl)
+{
+	return racl->names.count
+	     + (racl->user_obj != NO_ENTRY)
+	     + (racl->group_obj != NO_ENTRY)
+	     + (racl->mask_obj != NO_ENTRY)
+	     + (racl->other_obj != NO_ENTRY);
+}
+
 static int calc_sacl_entries(const rsync_acl *racl)
 {
 	/* A System ACL always gets user/group/other permission entries. */
@@ -574,6 +587,96 @@ int get_acl(const char *fname, stat_x *sxp)
 	return 0;
 }
 
+/* === OLD Send functions === */
+
+/* Send the ida list over the file descriptor. */
+static void old_send_ida_entries(int f, const ida_entries *idal, char tag_char)
+{
+	id_access *ida;
+	size_t count = idal->count;
+	for (ida = idal->idas; count--; ida++) {
+		if (tag_char == 'U') {
+			if (!(ida->access & NAME_IS_USER))
+				continue;
+			add_uid(ida->id);
+		} else {
+			if (ida->access & NAME_IS_USER)
+				continue;
+			add_gid(ida->id);
+		}
+		write_byte(f, tag_char);
+		write_byte(f, ida->access);
+		write_int(f, ida->id);
+	}
+}
+
+/* Send an rsync ACL over the file descriptor. */
+static void old_send_rsync_acl(int f, const rsync_acl *racl)
+{
+	size_t count = old_count_racl_entries(racl);
+	write_int(f, count);
+	if (racl->user_obj != NO_ENTRY) {
+		write_byte(f, 'u');
+		write_byte(f, racl->user_obj);
+	}
+	old_send_ida_entries(f, &racl->names, 'U');
+	if (racl->group_obj != NO_ENTRY) {
+		write_byte(f, 'g');
+		write_byte(f, racl->group_obj);
+	}
+	old_send_ida_entries(f, &racl->names, 'G');
+	if (racl->mask_obj != NO_ENTRY) {
+		write_byte(f, 'm');
+		write_byte(f, racl->mask_obj);
+	}
+	if (racl->other_obj != NO_ENTRY) {
+		write_byte(f, 'o');
+		write_byte(f, racl->other_obj);
+	}
+}
+
+/* Send the ACL from the stat_x structure down the indicated file descriptor.
+ * This also frees the ACL data. */
+void old_send_acl(stat_x *sxp, int f)
+{
+	SMB_ACL_TYPE_T type;
+	rsync_acl *racl, *new_racl;
+	item_list *racl_list;
+	int ndx;
+
+	type = SMB_ACL_TYPE_ACCESS;
+	racl = sxp->acc_acl;
+	racl_list = &access_acl_list;
+	do {
+		if (!racl) {
+			racl = new(rsync_acl);
+			if (!racl)
+				out_of_memory("send_acl");
+			*racl = empty_rsync_acl;
+			if (type == SMB_ACL_TYPE_ACCESS) {
+				rsync_acl_fake_perms(racl, sxp->st.st_mode);
+				sxp->acc_acl = racl;
+			} else
+				sxp->def_acl = racl;
+		}
+
+		if ((ndx = find_matching_rsync_acl(racl, type, racl_list)) != -1) {
+			write_byte(f, type == SMB_ACL_TYPE_ACCESS ? 'a' : 'd');
+			write_int(f, ndx);
+		} else {
+			new_racl = EXPAND_ITEM_LIST(racl_list, rsync_acl, 1000);
+			write_byte(f, type == SMB_ACL_TYPE_ACCESS ? 'A' : 'D');
+			old_send_rsync_acl(f, racl);
+			*new_racl = *racl;
+			*racl = empty_rsync_acl;
+		}
+		racl = sxp->def_acl;
+		racl_list = &default_acl_list;
+	} while (BUMP_TYPE(type) && S_ISDIR(sxp->st.st_mode));
+
+	free_acl(sxp);
+}
+
 /* === Send functions === */
 
 /* Send the ida list over the file descriptor. */
@@ -649,6 +752,11 @@ static void send_rsync_acl(int f, rsync_acl *racl, SMB_ACL_TYPE_T type,
  * This also frees the ACL data. */
 void send_acl(int f, stat_x *sxp)
 {
+	if (protocol_version < 30) {
+		old_send_acl(sxp, f);
+		return;
+	}
+
 	if (!sxp->acc_acl) {
 		sxp->acc_acl = create_racl();
 		rsync_acl_fake_perms(sxp->acc_acl, sxp->st.st_mode);
@@ -666,6 +774,160 @@ void send_acl(int f, stat_x *sxp)
 	}
 }
 
+/* === OLD Receive functions */
+
+static void old_recv_rsync_acl(rsync_acl *racl, int f)
+{
+	static item_list temp_ida_list = EMPTY_ITEM_LIST;
+	SMB_ACL_TAG_T tag_type = 0;
+	uchar computed_mask_bits = 0;
+	id_access *ida;
+	size_t count;
+
+	if (!(count = read_int(f)))
+		return;
+
+	while (count--) {
+		char tag = read_byte(f);
+		uchar access = read_byte(f);
+		if (access & ~ (4 | 2 | 1)) {
+			rprintf(FERROR, "old_recv_rsync_acl: bogus permset %o\n",
+				access);
+			exit_cleanup(RERR_STREAMIO);
+		}
+		switch (tag) {
+		case 'u':
+			if (racl->user_obj != NO_ENTRY) {
+				rprintf(FERROR, "old_recv_rsync_acl: error: duplicate USER_OBJ entry\n");
+				exit_cleanup(RERR_STREAMIO);
+			}
+			racl->user_obj = access;
+			continue;
+		case 'U':
+			tag_type = SMB_ACL_USER;
+			break;
+		case 'g':
+			if (racl->group_obj != NO_ENTRY) {
+				rprintf(FERROR, "old_recv_rsync_acl: error: duplicate GROUP_OBJ entry\n");
+				exit_cleanup(RERR_STREAMIO);
+			}
+			racl->group_obj = access;
+			continue;
+		case 'G':
+			tag_type = SMB_ACL_GROUP;
+			break;
+		case 'm':
+			if (racl->mask_obj != NO_ENTRY) {
+				rprintf(FERROR, "old_recv_rsync_acl: error: duplicate MASK entry\n");
+				exit_cleanup(RERR_STREAMIO);
+			}
+			racl->mask_obj = access;
+			continue;
+		case 'o':
+			if (racl->other_obj != NO_ENTRY) {
+				rprintf(FERROR, "old_recv_rsync_acl: error: duplicate OTHER entry\n");
+				exit_cleanup(RERR_STREAMIO);
+			}
+			racl->other_obj = access;
+			continue;
+		default:
+			rprintf(FERROR, "old_recv_rsync_acl: unknown tag %c\n",
+				tag);
+			exit_cleanup(RERR_STREAMIO);
+		}
+		ida = EXPAND_ITEM_LIST(&temp_ida_list, id_access, -10);
+		ida->access = access | (tag_type == SMB_ACL_USER ? NAME_IS_USER : 0);
+		ida->id = read_int(f);
+		computed_mask_bits |= access;
+	}
+
+	/* Transfer the count id_access items out of the temp_ida_list
+	 * into the names ida_entries list in racl. */
+	if (temp_ida_list.count) {
+#ifdef SMB_ACL_NEED_SORT
+		if (temp_ida_list.count > 1) {
+			qsort(temp_ida_list.items, temp_ida_list.count,
+			      sizeof (id_access), id_access_sorter);
+		}
+#endif
+		if (!(racl->names.idas = new_array(id_access, temp_ida_list.count)))
+			out_of_memory("unpack_smb_acl");
+		memcpy(racl->names.idas, temp_ida_list.items,
+		       temp_ida_list.count * sizeof (id_access));
+	} else
+		racl->names.idas = NULL;
+
+	racl->names.count = temp_ida_list.count;
+
+	/* Truncate the temporary list now that its idas have been saved. */
+	temp_ida_list.count = 0;
+
+	if (!racl->names.count) {
+		/* If we received a superfluous mask, throw it away. */
+		if (racl->mask_obj != NO_ENTRY) {
+			/* Mask off the group perms with it first. */
+			racl->group_obj &= racl->mask_obj | NO_ENTRY;
+			racl->mask_obj = NO_ENTRY;
+		}
+	} else if (racl->mask_obj == NO_ENTRY) /* Must be non-empty with lists. */
+		racl->mask_obj = (computed_mask_bits | racl->group_obj) & 7;
+}
+
+/* Receive the ACL info the sender has included for this file-list entry. */
+void old_recv_acl(struct file_struct *file, int f)
+{
+	SMB_ACL_TYPE_T type;
+	item_list *racl_list;
+
+	if (S_ISLNK(file->mode))
+		return;
+
+	type = SMB_ACL_TYPE_ACCESS;
+	racl_list = &access_acl_list;
+	do {
+		char tag = read_byte(f);
+		int ndx;
+
+		if (tag == 'A' || tag == 'a') {
+			if (type != SMB_ACL_TYPE_ACCESS) {
+				rprintf(FERROR, "receive_acl %s: duplicate access ACL\n",
+					f_name(file, NULL));
+				exit_cleanup(RERR_STREAMIO);
+			}
+		} else if (tag == 'D' || tag == 'd') {
+			if (type == SMB_ACL_TYPE_ACCESS) {
+				rprintf(FERROR, "receive_acl %s: expecting access ACL; got default\n",
+					f_name(file, NULL));
+				exit_cleanup(RERR_STREAMIO);
+			}
+		} else {
+			rprintf(FERROR, "receive_acl %s: unknown ACL type tag: %c\n",
+				f_name(file, NULL), tag);
+			exit_cleanup(RERR_STREAMIO);
+		}
+		if (tag == 'A' || tag == 'D') {
+			acl_duo *duo_item;
+			ndx = racl_list->count;
+			duo_item = EXPAND_ITEM_LIST(racl_list, acl_duo, 1000);
+			duo_item->racl = empty_rsync_acl;
+			old_recv_rsync_acl(&duo_item->racl, f);
+			duo_item->sacl = NULL;
+		} else {
+			ndx = read_int(f);
+			if (ndx < 0 || (size_t)ndx >= racl_list->count) {
+				rprintf(FERROR, "receive_acl %s: %s ACL index %d out of range\n",
+					f_name(file, NULL), str_acl_type(type), ndx);
+				exit_cleanup(RERR_STREAMIO);
+			}
+		}
+		if (type == SMB_ACL_TYPE_ACCESS)
+			F_ACL(file) = ndx;
+		else
+			F_DIR_DEFACL(file) = ndx;
+		racl_list = &default_acl_list;
+	} while (BUMP_TYPE(type) && S_ISDIR(file->mode));
+}
+
 /* === Receive functions === */
 
 static uint32 recv_acl_access(int f, uchar *name_follows_ptr)
@@ -787,6 +1049,11 @@ static int recv_rsync_acl(int f, item_list *racl_list, SMB_ACL_TYPE_T type, mode
 /* Receive the ACL info the sender has included for this file-list entry. */
 void receive_acl(int f, struct file_struct *file)
 {
+	if (protocol_version < 30) {
+		old_recv_acl(file, f);
+		return;
+	}
+
 	F_ACL(file) = recv_rsync_acl(f, &access_acl_list, SMB_ACL_TYPE_ACCESS, file->mode);
 
 	if (S_ISDIR(file->mode))
diff --git a/compat.c b/compat.c
--- a/compat.c
+++ b/compat.c
@@ -193,13 +193,6 @@ void setup_protocol(int f_out,int f_in)
 	if (protocol_version < 30) {
 		if (append_mode == 1)
 			append_mode = 2;
-		if (preserve_acls && !local_server) {
-			rprintf(FERROR,
-			    "--acls requires protocol 30 or higher"
-			    " (negotiated %d).\n",
-			    protocol_version);
-			exit_cleanup(RERR_PROTOCOL);
-		}
 		if (preserve_xattrs && !local_server) {
 			rprintf(FERROR,
 			    "--xattrs requires protocol 30 or higher"
diff -up a/proto.h b/proto.h
--- a/proto.h
+++ b/proto.h
@@ -3,7 +3,9 @@
 int allow_access(char *addr, char *host, char *allow_list, char *deny_list);
 void free_acl(stat_x *sxp);
 int get_acl(const char *fname, stat_x *sxp);
+void old_send_acl(stat_x *sxp, int f);
 void send_acl(int f, stat_x *sxp);
+void old_recv_acl(struct file_struct *file, int f);
 void receive_acl(int f, struct file_struct *file);
 void cache_tmp_acl(struct file_struct *file, stat_x *sxp);
 void uncache_tmp_acls(void);
